Hi JingoJagget!
The answer is:
Yes.
Here is one way to do this for health:
Create a simple texture pack and put PNG images called “hud_heart_fg.png” and “hud_heart_bg.png” into that. The first one is for the foreground (red heart) and the second one for the background (black heart, it symbolizes the “absence” of health). Both images must be of size 16×16. The half heart is automatically created from the full heart image, it is just cut in half in the middle. If you want a half heart which was is not based on the full heart image, then you are out of luck: This is not possible with the Better HUD mod.
Creating a texture pack is very simple: Just create a folder in the “textures” folder inside your Minetest user directory. I guess you can give that folder any name you want. Now put the heart images into that folder, start Minetest and select your very own texture pack. If you now start playing, you should now have your very own heart images (but only for the Better HUD mod). Note that this change applies only locally, this means only your HUD is changed, the HUD of other players is not affected, even if you are hosting a server.
Repeat the same for hunger and armor, the images names are “hud_hunger_fg.png”, “hud_hunger_bg.png”, “hud_armor_fg.png” and “hud_armor_bg.png”.
You can view the original textures of the Better HUD mod here:
https://github.com/BlockMen/hud/tree/master/textures